In the course of producing semiconductor wafers, it is necessary to grind a predetermined profile or bevel on the perimeter of the wafer. Typically, this step is carried out on a machine known as an edge grinder, which has a rotatable chuck onto which the wafer is placed. The wafer is held in place by a vacuum created between the wafer and the surface of the chuck. As the chuck is rotated, the edge of the wafer is carried past a spinning grinding head to create the desired profile.
Because the perimeter of the wafer is rotated past the grinding head by the chuck, it is critical that the wafer be positioned perfectly centered over the rotation axis of the chuck. If the wafer is positioned off-center, a "grind out" may occur, where the grinding head does not contact the entire perimeter of the wafer because of the eccentric position of the wafer. Any grind out on a wafer makes the wafer unusable and thus reduces overall production.
About once a month, or when the wafer size is changed or a grind out occurs, an operator must carry out a master centering procedure whereby the positioning of wafers on the chuck is readjusted. In a typical machine, wafers are placed on the chuck by a fine-aligner which carries the wafers from a staging area at the side of the chuck to the chuck. The fine aligner moves the wafer laterally from a staging area to a predetermined location which should be laterally centered over the chuck. The chuck moves the wafer transversely in the front/back direction to carry the wafer from where it is dropped by the fine aligner into the grinding station. For the wafer to be properly centered on the chuck, the lateral position of the fine aligner and the front/back location of the chuck both must be correct when the wafer is released by the fine aligner.
The master centering procedure requires the operator to measure the eccentricity of the placement of the wafer on the chuck. This is accomplished by monitoring the position of the perimeter of a precisely machined test plate as it is rotated on the chuck. The plate has the same dimensions as the wafer, but is precisely machined for use as a reference. If the plate is dropped off-center, the perimeter of the plate will move in and out as the chuck is rotated. By measuring the position of the edge of the plate at 90-degree increments of rotation, an operator can calculate how much off center the wafer is and in what direction. Once the front/back and lateral errors are known, the operator can adjust the front/back position of the chuck and the lateral drop point of the fine adjuster to restore proper centering.
Unfortunately, in the past, the master centering procedure has been rather time-consuming to complete because of the difficulty of gaining access to the perimeter of the wafer to make measurements. In particular, the fine aligner obstructs access from above and the chuck is enclosed by a basin surrounding the sides and bottom. As a result, it has been necessary to reopen the fine aligner and reposition the measuring instrument for each measurement. The measuring instrument is typically a dial indicator mounted to an articulated arm connected to a magnetic base secured to a brace disposed above the chuck. Multiple measurements are required because the corrections entered may not correlate exactly with the physical repositioning of the chuck and fine aligner due to play in the mechanisms involved and inaccuracies of the measurement process resulting from the length of the arm on the measuring instrument. Normally, several measurement/correction cycles must be carried out to obtain the correct centering. Since each cycle may take several minutes, the overall process often requires an hour or more of operator time to complete, including setup time. During this time, the machine is, of course, out of production .